Electrocoating

ABSTRACT

Methods and apparatus for electrocoating a conductive article from an aqueous coating bath by means of a direct current potential applied to the article to be coated through a liquid metal such as mercury. The mercury is contained at the bottom of an electrocoating vessel wherein an intermediate layer of insulating liquid between the liquid metal and the aqueous coating bath prevents direct contact of the bath with the liquid metal. The article to be coated extends through the intermediate layer to establish electrical contact with the liquid meal and an electrodeposited coating is applied to that portion of the article in contact with the aqueous coating bath. This process is useful for applying coatings to metals by electrolytic plating of metals, electrophoretic deposition of resins and by anodizing a metal article.

United States Patent [72] inventor Lowell G. Wise Cleveland, Ohio [21]Appl. No. 787,875 [22] Filed Dec. 30, 1968 [45] Patented Sept. 21, 1971[73} Assignee The Sherwin-Williams Company Cleveland, Ohio [54]ELECTROCOATING 7 Claims, 2 Drawing Figs.

[52] U.S.Cl 204/15, 204/23, 204/45, 204/52, 204/58, 204/18 1 204/250 [51l Int. Cl C23b 5/48 [50] Field of Search 204/15, 45, I8], 219, 56, 58,23, 250

[56] References Cited UNITED STATES PATENTS 2,745,803 5/1956 Leveque204/195 2,873,237 2/l959 Lambertonetal. 3,254,004 5/1966 Jackson et al.

ABSTRACT: Methods and apparatus for electrocoating a conductive articlefrom an aqueous coating bath by means of a direct current potentialapplied to the article to be coated through a liquid metal such asmercury. The mercury is contained at the bottom of an electrocoatingvessel wherein an intermediate layer of insulating liquid between theliquid metal and the aqueous coating bath prevents direct contact of thebath with the liquid metal. The article to be coated extends through theintermediate layer to establish electrical contact with the liquid mealand an electrodeposited coating is applied to that portion of thearticle in contact with the aqueous coating bath. This process is usefulfor applying coatings to metals by electrolytic plating of metals,electrophoretic deposition of resins and by anodizing a metal article.

ELECTROCOATING BACKGROUND OF THE INVENTION In the conventionalelectrocoating processes an electrically conductive article, usually abase metal, is immersed in a coating bath and biased with a directcurrent potential to create a coating on the surface of the article. Insuch processes, an electrical circuit is needed which is capable ofconducting current from the source of power to the article to be coatedthrough the coating bath and back to the power source This circuit isusually made by contacting the article with a solid electrical conductorand an electrode of opposite polarity is contacted with the coatingbath. In the usual process, the article is suspended in the coating bathby a rack and the tempo- 1 rary electrical contact to the article ismade through such rack. This arrangement requires some means ofattaching the article to the rack, and the material handling problems inachieving this support and circuit function consumes considerablemanpower. The racking is frequently performed by manual labor, and thecoating process is sometimes interrupted by dirt or coating material atthe point of connection which prevents proper electrical contact withthe wire to the coated. One approach to this problem has been the use ofa mercury pool which becomes a part of the electrical circuit forcontacting the wire In US. Pat. No. 3,254,004, a relatively complexsystem for handling small articles to be coated with metal oxides byelectrophoresis is described. In this process, the mercury contacts boththe ware and the coating bath. This arrangement limits the number ofelectrocoating processes in SUMMARY OF THE INVENTION In this invention,a liquid metal contact means, such as liquid mercury or gallium, iscontained in an electrocoating vessel and connected to a source of DCelectrical power. A liquid heavier than the aqueous coating bath andhaving high resistance to the passage of electrical current forms acovering layer over the liquid metal to insulate the liquid metal fromthe coating bath. An article to be electrocoated is immersed in theaqueous bath with an electrical path extending through the intermediateinsulating liquid layer to make electrical contact with the liquidmetal. An electrode of opposite polarity is immersed in the coating bathand connected to the power source to complete the circuit. Theintermediate insulating liquid forms a continuous layer between theliquid electrode and the coating bath, preventing chemical orelectrochemical interaction between these phases. That portion of theware which contacts the coating bath is coated anodically orcathodically while the portions of the ware in contact with the liquidmetal and insulating liquid remain uncoated. By shifting the position ofthe ware, the uncoated portions can contact the coating bath and uniformcoating can be obtained in some systems.

THE DRAWING FIG. 1 is a vertical cross section view of an electrocoatingtank showing an electrical circuit schematically; and

FIG. 2 is a vertical cross section view of a rotary coating vessel.

DESCRIPTION In FIG. 1, an electrocoating tank 10 contains a layer ofliquid mercury metal 12, which is electrically connected to a source ofdirect current potential, such as power source 14, through a solid metalplate 16. A thin layer of insulating liquid 20 is laid over the mercury12 and the aqueous electrocoating bath 22 is contained in the vessel 10over the insulating layer 20. A second electrode 24 is immersed in theelectrocoating bath 22 and connected to the DC power source 14.

The circuit is completed when an article 30 to be coated from the bath22 is placed in electrical contact with the liquid mercury 12, extendingthrough the intermediate insulating layer 20, the portion 32 of thearticle below the interface between the insulating layer and coatingbath is not electrocoated in this position. The portion of the articleextending through the insulating layer into the liquid mercury may beformed integrally with the article 30 or may be a separate conductorattached to the portion being coated. The electrocoating vessel 10 canbe constructed on an insulating material or a metal tank can be providedwith a layer of insulation in those portions exposed to theelectrocoating bath 22. The article to be coated can be carried throughthe tank 10 by a conveyor or other suitable handling equipment.

A second embodiment of the invention, especially adapted for coatingsmaller articles is shown in FIG. 2. A rotary electrocoating vessel 10Ain the shape of a cylindrical drum contains a body of liquid metal 12,an insulating liquid layer 20 and an electrocoating bath 22. Electricalcontact is made from the liquid metal 12 to the power source 14 throughan insulated conductor 15 and a movable brush device 17. An electrode 24is suspended in the bath 22. Articles 30A to be electrocoated are placedin the bath, with someportions extending through the insulating layer 20to establish electrical contact. The relative positions of the articles30A are shifted by rotation of the coating vessel 10A. After the desiredcoating is obtained, the articles are removed.

The insulating liquid can be chosen from a variety of chemical compoundsor mixtures of different compounds. This liquid should be substantiallyinert to the components of the aqueous coating bath as well as theliquid metal phase. A desirable insulating liquid is insoluble in theother liquids and has a specific gravity greater than the aqueouselectrocoating bath under the conditions of operation. Its boiling pointmust be higher than the operating temperature of the electrocoatingprocess and the preferred insulating liquids have a boiling 0 pointgreater than 70 C. To assure complete separation of the coating bath andinsulating liquid, the latter should have a specific gravity greaterthan l.l g./l. High electrical resistivity is needed and thoseinsulating liquids having an electrical resistivity greater than about10 ohm-cm. are suitable. The preferred insulating liquids include thehalogenated hydrocarbons such as bromobenzene, carbon tetrachloride,1,2- dichloroethane, l,l,l-trichloroethane and n-propyl bromide. Otherinsulating liquids include the nitro-substituted compounds and carbondisulfide, if compatible with the overall system. The thickness of theinsulating layer should be sufficient to prevent electrical contactbetween the liquid metal and electrocoating bath. Ordinarily this layercan be less than a millimeter; but if the lower portion of the articleis to be masked to prevent deposition of coating material, theinsulating layer can be thicker. At high DC potential, the electricalbreakdown of a very thin layer can become a problem and dielectricproperties should be considered for such applications.

The preferred liquid metal is mercury, although gallium can be used athigher temperatures. The amount of liquid metal is not critical. If ametal bottomed vessel is used, a thin film can be sufiicient for manysystems. The high specific gravity of theses metals will float manymetal articles being elec- 5 trocoated.

As examples of electrocoating baths which can be used in the practice ofthis invention, three types of aqueous baths will be discussed. Onetypical system is the electrolytic plating of metals, such as copper,from an aqueous solution of the metal salt, such as copper sulfate orcyanide. In these systems, the article to be coated is biasedcathodically and the metal cations discharge at the surface of thearticle to form the metal coating.

If the article to be coated is an oxidizable metal such as aluminum ortantalum, an oxide coating may be deposited by applying a positivepotential to the article and contacting the metal surface with anaqueous anodizing solution containing oxygen-forming anions, such ashydroxyl, sulfate or oxalate ions.

Another electrocoating bath which can be used is a waterdispersion of apolycarboxylic acid resin having an acid value greater than about 30 andbeing at least partially neutralized with a base. Such resins aredisclosed in U.Sv Pat. No. 3,230,162.

While the invention has been described by reference to specificexamples, there is no intent to limit the inventive concept except asset forth in the following claims.

1. A process for coating an electrically conductive article with coatingmaterial from an aqueous electrocoating bath comprising the steps of:

containing a liquid metal in an electrocoating vessel, said liquid metalcomprising mercury or gallium;

maintaining a layer of high-resistivity inert liquid in contact with theliquid metal electrode and an aqueous electrocoating bath toelectrically insulate the liquid metal electrode from the aqueouselectrocoating bath with an intermediate layer, said high-resistivityliquid having a specific gravity greater than the aqueous electrocoatingbath and less than the liquid metal;

immersing the article to be coated in the aqueous electrocoating bathwhile providing an electrical contact between the liquid metal electrodeextending through the intermediate insulating layer; and biasing theliquid metal electrode and article to be coated with a direct currentpotential to produce an electrochemically deposited coating on thearticle.

2. The process of claim 1 wherein the liquid metal consists essentiallyof mercury and the high-resistivity liquid consists essentially ofhalogenated hydrocarbons having a specific gravity greater than 1.1g./l., a boiling point higher than 70 C. and an electrical resistivitygreater than l0 ohm-cm.,

3. The process of claim 1 wherein the metal electrode is biasedanodically and the aqueous electrocoating bath comprises a dispersion ofa polycarboxylic acid resin having an acid value greater than about 30and being at least partially neutralized with a base.

4. The process of claim 1 wherein the liquid metal electrode and articleare biased anodically and the aqueous coating bath consists essentiallyof an electrolytic solution containing oxygen-forming anions.

5. The process of claim I wherein the liquid article comprises aluminumand the electrochemically deposited coating consists essentially ofaluminum oxide.

6. The process of claim 1 wherein the liquid metal electrode and articleare biased cathodically and the aqueous electrocoating bath consistsessentially of an electrolytic solution of metal ions.

7. ln electrocoating apparatus wherein an electrically conductivearticle in contact with an aqueous coating bath is biased with a directcurrent potential through electrical contact with a liquid mercurycontact means, the improvement which comprises:

a high-resistivity liquid disposed in a continuous intermediate layerbetween the aqueous coating bath and the liquid mercury contact meanswhereby an electrical path is provided from the liquid mercury to theaqueous bath through the conductive article extending through thehigh-resistivity liquid and whereby liquid mercury is electricallyinsulated from direct contact with the aqueous coating bath by the layerof high-resistivity liquid.

2. The process of claim 1 wherein the liquid metal consists essentiallyof mercury and the high-resistivity liquid consists essentially ofhalogenated hydrocarbons having a specific gravity greater than 1.1g./l., a boiling point higher than 70* C. and an electrical resistivitygreater than 107 ohm-cm.,
 3. The process of claim 1 wherein the metalelectrode is biased anodically and the aqueous electrocoating bathcomprises a dispersion of a polycarboxylic acid resin having an acidvalue greater than about 30 and being at least partially neutralizedwith a base.
 4. The process of claim 1 wherein the liquid metalelectrode and article are biased anodically and the aqueous coating bathconsists essentially of an electrolytic solution coNtainingoxygen-forming anions.
 5. The process of claim 1 wherein the liquidarticle comprises aluminum and the electrochemically deposited coatingconsists essentially of aluminum oxide.
 6. The process of claim 1wherein the liquid metal electrode and article are biased cathodicallyand the aqueous electrocoating bath consists essentially of anelectrolytic solution of metal ions.
 7. In electrocoating apparatuswherein an electrically conductive article in contact with an aqueouscoating bath is biased with a direct current potential throughelectrical contact with a liquid mercury contact means, the improvementwhich comprises: a high-resistivity liquid disposed in a continuousintermediate layer between the aqueous coating bath and the liquidmercury contact means whereby an electrical path is provided from theliquid mercury to the aqueous bath through the conductive articleextending through the high-resistivity liquid and whereby liquid mercuryis electrically insulated from direct contact with the aqueous coatingbath by the layer of high-resistivity liquid.